I've tryed my hand at shooting birds. I don't have experience at this one and, after harassing the crows that live happily in front of my building, I've reached the conclusion that there are too many things that I don't know on the subject.
I've used a Tamron 28-300 and and the EF 100/2.8 macro, none has IS.
It was late afternoon and I've chosen ISO 400, TV set at 1/500 and aperture came out at 6.3 for the 28-300 and 2.8 for the 100 mm( widest for each lens).
RAW, AF-servo, single point focusing, continuous shooting drive.
Appart from my personal record of about 300 shots in about an hour, the rezult is lousy.
I know that the lenses I use are not the best ones on the subject( the 28-300 is an old one, soft is it's middle name, and slow focusing, and the EF100 is, well, 100). But doing the right thing I should have had at least one decent shot.
Annoyng noise, the highest I've managed with this camera, out of focus, and obviously 1/500 is not enough to catch a bird in flight ( but I may be wrong, of course).
One other thing is the difficulty to switch swiftly from binocular view( i.e. my eyes) to monocular one -the camera- and catch the bird in flight. But this may get better with practice.
I know I'm doiong everything wrong, I like to learn from experience, but I'd like also to have some pointers in the right direction.
Thanks

The images look somewhat under-exposed. Some random tips that work for me are:

Don't be afraid to use higher ISO. I left the 50D on auto-ISO most of the time when I had it.

Get the shutter speed up if you can. I target 1/1000 in good light, depending on the speed and effect desired. At that speed, IS doesn't matter. You will probably still get blurred wingtips at that speed. I tend to use shutter priority which will generally leave the lens wide open and minimise needed ISO. Alternatively you can use aperture priority and fixed high ISO to get the shutter speeds into desired range.

Having said that, if you can pan well, you can use longer shutter speeds and still get a degree of sharpness with some artistic blur too.

On the 50D, I tend to use centre point AF only. The camera was too prone to pick something at random if you use more than one point. It does mean you have to get good at tracking the bird with that point, and recompose by crop afterwards.

I shoot jpeg as you can do much longer continuous bursts than with raw.

And it is very normal to have a relatively low % of successful shots when it comes to random flying birds, unless you are working to a very predictable pattern.

I'll do some more crows soon In the meanwhile I'm saving for a 70-200
That one should help too.
I'm thinking to try to shoot the fruit bats that live in a park behind my house.
The problem is those are really hard to shoot. Almost complete darkness and one can't see them if not in flight. Need a big flash gun I think.

I have been playing around with bird photography amongst other things for a while and I believe that patience, practice/ experience and a little luck are probably the main things to have - probably above which camera gear you have. (I am not knocking your gear - far from it - I use a Sigma 70-300 a lot and have some M42 lenses I just love using - the m42 lenses are hardly up to date top notch gear but they are still pretty good)

I too have been trying to work out how to take better bird photo's and it seems to be the age old issue of light in may but not all cases, I am trying to experiment with having the sun behind me shining on to my subject as this seems to be one of the easiest things to forget amongst what ISO/ F stop/ shutter speed etc when a well let subject can even mask some errors or make up for my incompetencies.

I am sorry that this is a little non specific and lets face it a well lit subject is not always attainable but when it is it all seems to come together

Below is a link to my Flickr account, the in flight shots are with the Sigma, the more static ones are often but not exclusively with an M42 lens.

Next I plan to practice capturing birds in flight with my M42 - manual focus lenses.

Hope this helps, an example is when I have stood on the wrong side of a canal taking photo's of ducks and struggling when merely crossing the bridge and taking the other 'well lit' side has made a massive difference.

Have a go in ISO 400 and aperature priority.
The first shot appears to be pretty low light,it'll be tough to get a goodun in low light.
It appears you were panning the one with the car in the background...great job on that!!

As others have mentioned, the keys to good bird photography are skill, light, and lens...not necessarily in any particular order. Mother nature controls the light, so you either have to wait for better light when it wants to cooperate, or find ways to get around the problem through better lenses and more skill (higher ISO, intentional underexposure with post-processing, etc). If you have a poor lens, really good light and some decent skill can get you by. If you have mediocre skill, a day with excellent light and a really nice lens can get you decent results. But to consistently get good results, it's best to have all three of these things!

Another thing many don't realize is how much patience, how much time, and how many shots many bird photographers commit to getting good results. 300 shots in an hour sounds like a lot - but I've seen bird photographers come home with 3,000 shots in a few hours. As you get better, you don't need to take as many shots to land good ones - you start to get instincts about the right settings for the situation, your tracking gets better as you anticipate the movements of different birds, and you get familiar with your camera's settings to where you can make 3 or 4 different settings adjustments on the fly in just a second or two (ISO, aperture, focus point, EV, for example).

If you get a better and faster lens, you will see much improvement - but you should still be able to practice your tracking, focus, and metering with the lens you've got - you may not have the sharpest details in the feathers, but landing a perfect exposure and getting the focus in the right spot is so much more important.

Try to shoot on nice, sunny, cloudless days if you can - and best when a little earlier or later in the day, rather than the middle of the afternoon. The good light will make a vast difference with your lens, and give you a much higher hit rate. It looks like you did OK with the tracking of the flying bird - especially with a distracting background, that was pretty good to keep focus on the bird. A larger aperture will help in these situations, to narrow the depth-of-field and help keep backgrounds blurrier and less distracting. AF-C mode may help if you weren't already using it - but you may have to watch which focus points to use - if using a wide or multi-point focus in AF-C, the camera may occasionally miss focus on a small moving bird and instead grab focus on the background, if the bird is flying in front of some branches or trees - in these cases, switching to a smaller focus point or center spot focus will allow you to put the crosshairs on the bird to tell the focus where to go - but you need to get really proficient with panning on the bird. Against a blue sky or clear background, multipoint focus will usually get the job done.

In your samples there is definitely some underexposure hurting you - higher ISO likely would be needed - noise will result, which can be reduced in post-processing...but it also looks like you were shooting on a fairly dim day. Also, I don't know what metering mode you were using - I'd recommend trying center-weighted metering rather than multi-point or wide metering - this will allow the camera to better meter for the smaller subject of the bird, putting more emphasis on the center metering circle or area. Spot meter can be too specific, and be very hard to get right shooting something in motion, because the metering area is so small that just different colored feathers on the neck can cause wildly different exposures. Center weight usually lands me the best exposures. As your camera seems to be underexposing a lot, make sure you had no -EV dialed in, and maybe even consider adding a little +EV, like .3, if even in good light you are tending to get slightly underexposed results.